U.S. government health officials say they believe the West Nile virus can be transmitted through blood transfusions. This is the third year the virus has been present in the United States. This year, more than 1,600 people have been infected and at least 80 have died.

Until this year, it was thought West Nile virus could only be transmitted through mosquito bites. But, this year a woman in Mississippi contracted the virus after receiving blood from an infected donor, and in Georgia, donated organs spread the virus to four recipients. Dr. Jesse Goodman of the U.S. Food and Drug Administration says it is probable the virus can be spread through blood.

"We really believe that given the results of these recent investigations, it is most prudent to assume that blood-borne transmission can and likely has occurred," he said.

Dr. Goodman says the government is working with medical research firms to develop a test to screen donated blood for West Nile virus. He is not sure how long it will take to develop such a test.

"What we are trying to do here is jumpstart and facilitate this process so that we can get a test available as soon as possible to screen blood if that is needed," he said.

Most people who contract West Nile virus do not get sick. Those who do are usually older people or people already ill. In serious cases, the virus causes a potentially deadly inflammation of the brain. A new study published Thursday also warns doctors that West Nile virus can also cause polio-like paralysis in some cases.

Doctors also say the number of new cases of West Nile virus appears to have peaked in the southern United States, and either has peaked or will soon in the northern part of the country. West Nile virus has been reported in 36 states and Washington, D.C. The state with the most cases is the Midwest state of Illinois. More than 450 cases have been reported there, and at least 23 people have died from West Nile-related illness. Most of Illinois' cases are in the Chicago area.

State officials expect to stop seeing new cases in a few weeks, when the first freezing temperatures of autumn kill most of the area's mosquitoes.